1. Field of the Invention
The present invention relates to an alignment method for an ophthalmic measurement apparatus in which alignment of a measuring optical system is required for an eye to be examined of a patient, and an alignment device thereof.
2. Description of the Related Art
As an alignment method for an ophthalmic measurement apparatus equipped with an automatic alignment function, several alignment methods have been conventionally known.
There is known an apparatus in which alignment is manually performed that a front eye portion of an eye to be examined is image-captured by an imaging device of an imaging optical system, the front eye portion of the eye to be examined is displayed on a display device such as a monitor television or the like by the image signal from the imaging device, the imaging optical system is moved horizontally and vertically by operating an operation lever such as a joy stick or the like to manually move the center of a pupil of the eye to be examined toward the center of the imaging device being the center of the display device, and then, the imaging optical system is operated to move back and forth by the operation lever, and a working distance between the eye to be examined and the imaging optical system is adjusted so as to make the front eye portion of the eye to be examined sharp.
In this manual alignment, since the work of matching the optical axis of the imaging optical system with the cornea vertex (visual axis) of the eye to be examined is performed based on examiner's intuition, an alignment work is not easy. As a device for solving the problem, there is known an automatic refractometer, a tonometer or the like, where index light is projected onto the cornea of the eye to be examined so as to form a bright spot image (index image) at the half position of a radius of curvature of the cornea, the bright spot image is formed on a two-dimensional light-receiving device such that this bright spot image becomes the cornea vertex of the eye to be examined, a photographing optical system is driven horizontally and vertically by three-dimensional driving device to allow the bright spot image to move toward the center of the imaging device, and automatic alignment is performed.
However, in this automatic alignment constitution, a region where a light-receiving device can detect the bright spot image formed by reflected light from the cornea is narrow, there is a room for improvement.
Specifically, when the bright spot image comes off from a detection region where automatic alignment can be performed and reflected light cannot be detected in the initial state, it is necessary that an examiner manually operate to move the imaging optical system via the operation lever (joy stick) horizontally and vertically to perform rough alignment in a direction where the bright spot image matches the approximately central portion of the light-receiving device until the measuring optical system (ophthalmic unit) comes in a detection region with respect to the eye.
This causes a load to the examiner operating the system and could prevent quick measurement. As one solution to this problem, a method of expanding a detection region of alignment by using a sensor having a large aperture light-receiving lens or wide light-receiving plane is considered. However, such a solution could result in an undesirable result such as a larger ophthalmic measurement apparatus and higher cost/complicatedness of the system.
Therefore, as an alignment device of an ophthalmic measurement apparatus for solving the problem, a system is also known where the above-described bright spot image is formed on the imaging device of the imaging optical system as a central bright spot image, four index lights are projected onto the pupil rim of the cornea of the eye to be examined, four bright spot images from the pupil rim are formed on the imaging device as rim bright spot images, and when one of the rim bright spot images is formed on the imaging device, imaging optical system is driven to operate horizontally and vertically by a three-dimensional driving device to perform automatic alignment such that the remaining three bright spot images are formed on the imaging device based on the rim bright spot images, and a detection region in horizontal and vertical (XY) directions is expanded (for reference, see Japanese Patent No. 3606706).
However, even in the alignment method using four rim bright spot images, automatic alignment does not start until one of the rim bright spot images is formed on the imaging device, rim bright spot images come out of focus and do not become sharp when the working distance between the cornea of the eye to be examined and the imaging optical system is largely shifted, and automatic alignment does not start even if the rim bright spot images are formed on the imaging device.